1. Field of the Invention
The present invention relates to image forming apparatuses and image forming methods that use transparent toner.
2. Description of the Related Art
Recently, image forming apparatuses using a transparent glossy medium as a toner have been practically realized (Japanese Patent Application Laid-Open No. 2007-199209). For example, by using a transparent toner over the whole surface of a printed product, gloss can be improved and a coating (protective) effect can be obtained.
Further, by applying a transparent toner to a specific character or the shape of a graphic, a printed product which better reflects a user's intentions can be generated.
On the other hand, page description languages (PDLs) that can give a transparent effect to an object exist.
For example, in Adobe's PDF format (PDF Reference, 5th Edition, Version 1.6, Adobe Systems Incorporated, ISBN: 0-321-30474-8), a transparent effect is imparted between a background object and a transparent object, so that an object given a transparent effect can be displayed on a display and printed.
Now, the types and characteristics of transparent films, such as colored transparent cellophane and plastic shopping bags, will be discussed.
In these substances, as the transmittance becomes higher, the glossiness becomes higher. On the contrary, as the transmittance becomes lower, the glossiness becomes lower.
FIG. 10 is a schematic diagram illustrating a situation where an object is observed via a transparent substance.
In FIG. 10, incident light 103 is diffusely reflected at the surface of the transparent substance 101 in proportion to the smoothness (flatness) of the surface, whereby the transmittance decreases.
That portion of the incident light 103 which is not diffusely reflected at the surface of the transparent substance 101 passes through the transparent substance 101, and is incident on a white object 102. This light is reflected by the white object 102, passes through the transparent substance 101, and is then incident on an observer 104. The transparent substance 101 absorbs a portion of the light that passes through it 101. As a result of such absorption and diffuse reflection, the white object 102 is perceived by the observer 104 as being a dusky red color.
However, an image forming apparatus which can simulate such a transparent substance has heretofore not been achieved.
Here, the case in which the transparent effect in current PDLs is printed on a paper surface using electrophotographic technology is considered.
In current PDLs, an object having the transparent effect and a transparent toner are not associated.
Therefore, printing is performed on the printed product using the process color toners, such as cyan, magenta, yellow, and black (CMYK), even if the object has a transparent effect.
Accordingly, there is a problem that regardless of whether each object has a transparent effect or not, the gloss on all of the objects is similar.
Further, to apply a transparent toner for expressing glossiness on a designated region, the user of the PDL must intentionally designate application of the transparent toner on objects which have a transparent effect.
That is, for example, a plate called a spot color plate, which is separate from the process colors, has to be prepared, and the objects on which the transparent toner is to be applied have to be designated on that plate.
In an image model based on RGB color space, for example, such as GDI+ in Microsoft's Windows®, spot colors other than the process colors cannot be handled.
Therefore, in order for the PDL user to apply a transparent toner, separate PDL data has to be generated using a different technique, such as form overlay. Further, to apply the transparent toner on a form overlay, substantial changes have to be made to image forming apparatuses, printer drivers, and applications, respectively. Further, the operations carried out by a user are complex.
In an image model based on CMYK color space, such as Adobe's PDF format, a spot color plate can be handled.
By applying a transparent toner to this spot color, it is much easier to designate the transparent toner than in the RGB image model.
FIGS. 11A and 11B are schematic diagrams illustrating image processing in a conventional image forming apparatus. The problems in a PDL having a spot color plate will now be described with reference to these drawings.
In FIG. 11A, in the PDL, a colorless transparent object 201 overlaps a background, non-transparent object 202.
However, in image formation performed using a conventional PDL, generally, when objects overlap, based on an assumption that all of the objects are non-transparent, the object in the background is blocked out, which is called “knocked out”.
Therefore, unless overprinting is intentionally designated for the object 201, the object 202 in the background of the spot color plate is blocked out by the object 201, so that the background object 202 is deformed like object 203. As a result, as illustrated in FIG. 11B, there is a problem that the printing result is different from the shape intended by the user.
In color image forming apparatuses using electrophotographic technology, an image is formed using toners.
Thus, for example, if the respective CMYK process colors all have a 100% density and are arranged overlapping each other at the same location, problems arise such as insufficient fixing of the toners and toner scattering, and this can even lead to the image forming apparatus being damaged.
To prevent this, a toner density adjustment is generally performed, in which the toner total amount which can be printed overlapping at the same location is restricted.
FIG. 12 is a schematic diagram illustrating the toner density adjustment processing in a conventional color image forming apparatus.
FIG. 12 illustrates an object 301, which may be the number zero, for example, drawn in green (C=100%, Y=100%) with a 100% coating applied around the outline by a transparent toner.
An image portion 302 schematically illustrates a state where toner is applied at the locations illustrated by the dashed line. Since the image portion 302 is applied with transparent toner at a 100% density over the C=Y=100%, the toner total amount is 300%.
Thus, if the image portion corresponding to the object is thermally fixed onto the paper surface by the image forming apparatus by electrophotography, like the image portion 303, the toner may break up and scatter, which can lead to insufficient fixing and damage the apparatus.
To prevent this, in the toner density adjustment, for example, the upper limit of the toner total amount is limited to 150% for a total input of 200% of green C=Y=100%. As a result, generally, by reducing each of the channels in an equal ratio, such as C=Y=75%, the toner total amount is reduced without causing a change in color.
FIG. 13 is a schematic diagram illustrating the printing results when a toner density adjustment is performed based only on the densities of the channels corresponding to the respective colors in the image forming apparatus.
In FIG. 13, the colored objects 402 and 403 are objects having colors of respectively C=M=90% and C=Y=90%.
Now, assume that a 100% density transparent toner object 401 is overlapped thereon. When the toner density adjustment is performed on such printing data based only on the densities of each channel, only the overlapping portion 404 of the transparent toner 401 and the colored object 403 is subjected to a toner reduction. As a result, the portions in which the transparent toner does not overlap are printed without color modification.
Similarly, only the overlapping portion 405 of the transparent toner 401 and the colored object 402 corresponding to a normal object is subjected to a toner reduction. As a result, the portions in which the transparent toner does not overlap are printed without color modification.
When a transparent toner is used for protection in electrophotographic technology, even if an attempt is made to print objects having different toners by exactly aligning them, due to inaccuracies in the printing apparatus, the printing cannot always be performed with the objects correctly aligned (color misregistration).
FIGS. 14A and 14B are diagrams illustrating the color misregistration, which occurs when a non-transparent toner and a transparent toner are overlapped in an image forming apparatus.
In FIG. 14A, when a non-transparent toner object 502 and a transparent toner object 501 having exactly the same shape are printed, color misregistration occurs with the misregistration amount 503.
Conventionally, trapping has been used as a technique for preventing color misregistration.
As illustrated in FIG. 14B, trapping involves the following processing so that gaps between adjacent non-transparent objects cannot be seen.
More specifically, one of the objects is enlarged (in FIG. 14B, as illustrated by the arrow, the object on the right side is enlarged) and covers a part of the other object so that the gap is not noticeable. As a result, the same-shaped objects can be misaligned, and thus trapping is not an effective technique for situations where the protective effect is lost.